Field of the Invention
Disclosed herein is a substrate processing method for forming a through hole in a silicon substrate. In particular, the present disclosure relates to a method of manufacturing a substrate for a liquid discharge head, the method including forming a liquid supply port in a silicon substrate that is to be used as a substrate of a liquid discharge head that discharges liquid.
Description of the Related Art
A method using reactive ion etching (hereinafter referred to as RIE), which is a type of dry etching, is a known example of a substrate processing method for forming a through hole in a silicon substrate. Unlike wet etching, RIE uses an etching gas, and thus, RIE is suitable for forming a through hole perpendicular to a surface of a silicon substrate. Therefore, in the case of forming, in a substrate of a liquid discharge head, a representative example of which is an ink jet head, a through hole perpendicular to a surface of the substrate as a liquid supply port, RIE may be used. A method disclosed in U.S. Pat. No. 7,837,887 (see FIGS. 6 to 7) (hereinafter referred to as U.S. Pat. No. 7,837,887) is an example of a method for forming a liquid supply port in a substrate for an ink jet head by using RIE.
According to U.S. Pat. No. 7,837,887, a liquid supply port is formed in a substrate for an ink jet head through the following Steps 1 to 8. First, a silicon substrate having two opposing surfaces (first and second surfaces) is prepared (Step 1). Next, a plurality of first holes are formed in the first surface of the silicon substrate by performing an etching operation (Step 2). The first holes are filled with a photoresist (Step 3). After that, a flow-path-structure portion that includes a liquid discharge port and a liquid flow path is formed on the first surface (Step 4). Then, a second hole is formed in the second surface of the silicon substrate by performing an etching operation in such a manner that the second hole reaches the photoresist in the first holes (Step 5). In addition, oxygen plasma etching is performed on the second surface in such a manner as to remove part of the photoresist in the first holes via the second hole, so that end portions of side walls of the first holes, the end portions being continuous with the second hole are exposed (Step 6). Next, the shapes of the exposed end portions of the side walls of the first holes are corrected (Step 7). Finally, the photoresist with which the first holes have been filled is removed (Step 8), and the liquid supply port is completed.
In the above-described Steps 2 and 5, the holes are formed in the substrate by performing dry etching called RIE. In the dry etching, a reaction gas is introduced into a processing chamber and transformed into plasma, and one of the surfaces of the substrate, which is to be processed, is etched by using the reaction gas, which has been transformed into plasma, so that a hole having a predetermined shape is formed in the surface to be processed. More specifically, an upper electrode and a lower electrode that are connected to each other via a high-frequency power source are disposed so as to face each other in the processing chamber, and in a state where the substrate is fixed to the lower electrode by, for example, an electrostatic chuck, the reaction gas is supplied to a space between the lower electrode and the upper electrode from micropores formed in the upper electrode. The reaction gas is transformed into plasma between the upper electrode and the lower electrode, and the reaction gas, which has been transformed into plasma, etches the substrate.
U.S. Pat. No. 7,837,887 discloses a method for forming a liquid supply port by forming first holes, each of the first holes having such a depth that it does not extend through a substrate, in a first surface of the substrate first, and after that, forming a second hole in a second surface of the substrate on the opposite side to the first surface in such a manner that the second hole is in communication with the first holes. In addition, regarding the formation of the second hole, U.S. Pat. No. 7,837,887 discloses that, in the case where the second hole is formed in such a manner that the entirety or portions of open ends of the first holes are present within the second hole, unetched portions are formed in opening edge portions of the first holes, the opening edge portions being continuous with the second hole, and this results in formation of burrs called crowns.
The mechanism of occurrence of crowns will now be described with reference to FIGS. 6A to 6F.
FIG. 6A is a plan view illustrating a portion of a first surface 11a of a substrate 11 and illustrates two cases [i] and [ii] where the position of a first hole 16 with respect to a second hole 17 is different. In the case [i], a liquid supply port has a shape in which the entirety of an opening end of the first hole 16 is present within the second hole 17. In the case [ii], the liquid supply port has a shape in which a portion of the opening end of the first hole 16 is present within the second hole 17. A process of manufacturing the liquid supply port in each of the cases [i] and [ii] is illustrated on a time-series basis in FIGS. 6B to 6F by using a sectional view taken along line C-C′ of FIG. 6A and a sectional view taken along line D-D′ of FIG. 6A. In particular, FIG. 6B illustrates a state immediately before the first hole 16 and the second hole 17 are made to communicate with each other, and FIGS. 6C to 6E are enlarged views of a portion VIC TO VIE (the inside of a one dot chain line frame) in FIG. 6B.
In the case where the liquid supply port in each of the cases [i] and [ii] illustrated in FIG. 6A is formed, as illustrated in FIGS. 6B and 6C, there is a case where the first hole 16 is formed in the first surface 11a of the substrate 11 first, and after that, dry etching is performed on a second surface 11b of the substrate 11 in the direction of arrow 20 (hereinafter sometimes referred to as etching direction 20) in such a manner as to form the second hole 17 and make the second hole 17 to communicate with the first hole 16. In this case, in order to form the second hole 17 perpendicularly to the second surface 11b of the substrate 11, an etching method and in which a step of etching a substrate and a step of protecting a side surface of a hole formed by the etching are repeated, the method being so-called a Bosch process, is used. In other words, etching is performed several times, and before each etching, a process of forming protective films on the inner side surfaces of the holes 16 and 17 by using a coating gas is performed. It is known that, in the case where the Bosch process is performed, as illustrated in FIGS. 6C to 6E, surfaces of side walls 16a and 17a of the holes 16 and 17 are each formed in a shape having projections and depressions, the shape being called a scalloped shape. Note that bold lines in FIGS. 6C to 6E indicate the protective films.
When the second hole 17 is formed by the etching using the Bosch process, which has been described above, the protective film on a bottom portion 17b of the second hole 17, which is present in the etching direction 20, is etched, and then the bottom portion 17b is etched. In this case, the side wall 17a of the second hole 17 is a surface oriented in a direction perpendicular to the etching direction 20 and is covered with the protective film, so that it is not likely that the side wall 17a will be etched. As a result, the second hole 17 is formed while its opening dimension is maintained. After the second hole 17 has reached the first hole 16, as illustrated in FIGS. 6D and 6E, the etching operation for forming the second hole 17 is performed for some period of time in order to make the holes 16 and 17 to communicate with each other with certainty. Since the coating gas is introduced into the second hole 17 before the etching operation is performed, the protective film is formed not only on the side wall 17a of the second hole 17 but also on the side wall 16a of the first hole 16 that is in communication with the second hole 17. In this state, when the etching operation is performed in the direction of arrow 20, the protective film on the bottom portion 17b of the second hole 17, which is present in the etching direction 20, is etched, and then, the bottom portion 17b is etched. However, the side wall 16a of the first hole 16, which is present in a direction perpendicular to the etching direction 20, is etched to only a small extent. As a result, an unetched portion is formed in an edge portion of the side wall 16a of the first hole 16, the edge portion being continuous with the second hole 17, and a burr such as that illustrated in FIG. 6E, which will be referred to as a crown 21, is formed.
Note that, although such a crown is notably formed in the Bosch process, in which a side wall of an etching hole is protected before an etching operation is performed as described above, the formation of such a crown is not limited to the Bosch process. In other words, even in the case where a by-product, which is produced during the period when an etching operation is performed so as to form a hole, is attached to a side wall of the hole in such a manner as to function as a protective film, there is a possibility that a crown will be formed as in the above-described case where a side surface of a hole is protected.
In the case where a burr called a crown is formed within a liquid supply port as described above, the burr causes the following problems as described in U.S. Pat. No. 7,837,887. That is to say, the probability of the burr becoming an obstacle to the flow of liquid within the liquid supply port and affecting discharge characteristics increases. In addition, if the burr comes off and becomes mixed into the liquid as a foreign object, it becomes difficult to efficiently maintain the quality of an image printed by an ink jet head.
In order to solve these problems, in U.S. Pat. No. 7,837,887, it has been proposed to perform Steps 6 and 7, which are described above, that is, to process a burr formed within a liquid supply port.
However, in this proposal, when Steps 6 and 7 are performed, a process of protecting the interior of the liquid supply port excluding the burr with a resist or the like is required, and the overall process becomes complex, which in turn leads to an increase in the manufacturing costs of an ink jet head.